In the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. Drying groups applying a twin-wire draw include two wires which press the web, one from above and the other one from below, against the heated cylinder faces of the drying cylinders. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws. The free draws are susceptible to fluttering which may cause web breaks, in particular since the web is still relatively moist and, therefore, has a relatively low strength. For this reason, in recent years, increasing use has been made of a single-wire draw in which each group of drying cylinders has only one drying wire. The web runs on support of the single drying wire through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces thereof, and whereas, on the reversing cylinders or rolls arranged between the drying cylinders, the web remains at the side of the outside curve. Thus, in a single-wire draw, the drying cylinders are placed outside the drying wire loop and the reversing cylinders or rolls are situated inside the drying wire loop.
In prior art normal groups having a single-wire draw, the heated drying cylinders are typically arranged in an upper row and the reversing cylinders are therefore arranged in a lower row, below the upper row. The upper row and lower row are generally horizontal and parallel to one another. The current assignee's Finnish Patent No. 54,627 (corresponding to the current assignee's U.S. Pat. No. 4,202,113, the specification of which is hereby incorporated by reference herein) describes an arrangement wherein normal groups having a single-wire draw and so-called inverted groups having a single-wire draw are arranged one after the other. In typical inverted groups, e.g., of the type shown generally in U.S. Pat. No. 4,202,113, the heated drying cylinders are arranged in the lower row and the reversing suction cylinders or rolls are arranged in the upper row. This arrangement utilizing normal and inverted groups enables a principle objective to be achieved, i.e., to dry the web symmetrically from both of its sides.
With respect to additional prior art, reference is made to published International Patent Applications WO 88/06204 and WO 88/06205 (assigned to Beloit Corp.) which describe dryer sections.
Accordingly, in the following description, the terms "normal (drying) group" and "inverted (drying) group" are used to denote the cylinder groups having a single-wire draw as described above, as such is accepted terminology to those skilled in the art. The expression "single-wire draw" is equivalent to the terms "single felting" and "single tier" which are interchangeably used in the art. Similarly, the expression "twin-wire draw" is synonymous with the expression "double felting" as used in the art. Also, in the following description, the term "wire" when used to denote a wire in the dryer section or finishing section encompasses other types of dryer section clothings such as fabrics, which are more common today than wires, and felts.
In dryer sections that comprise inverted and normal drying groups, various problems have occurred. The present invention is directed toward a resolution of these problems. For example, problems have been encountered in the runnability of the dryer section and in the threading of the web, problems arising from differences in the speeds of different wires, problems in the removal of broke especially in inverted groups, as well as problems related to the control of transverse shrinkage of the web. These problems tend to become worse as the running speed of the paper machine becomes higher. As to the problems of control of the transverse shrinkage of the web, the use of a single tier dryer section in general provides better control than a dryer section applying double felting.
With respect to prior art involved in and related to the present invention, reference is made to the following patent publications and articles published in journals:
W. Haessner, "Trocknungstechnik und deren Entwicklung"; Das Papier 44, 10A, 1990; PA1 "The Valmet Sym-Run Concept", Paper Asia, May/Jun 1992; PA1 J. Yli-Kauppila, "Dryer Section for High Speed Paper Machines", Proceedings of the Helsinki Symposium of Alternate Methods of Pulp and Paper Drying, Helsinki Jun. 4-7, 1991; PA1 Sam Palazzolo, "No-draw drying", Tappi Journal, September 1990; PA1 W. Leitenberger, "Die Contirun-Trockenpartie fur schnellen, sicheren Bahnlauf", Das Papier, Heft 6, 1992; PA1 U.S. Pat. Nos. 3,753,298, 3,868,780, 4,602,439, 4,972,608, 4,982,513, 5,022,163, 5,065,529, 5,146,696, and 5,177,880; PA1 V. Korhonen and A. Kuhasalo, "Ropeless tail threading from press to reel", World Pulp & Paper Technology 1993; PA1 H. Lepisto und P. Eskelinen, "Verbesserung der Lauffahighkeit schneller Papiermaschinen mit Hilfe neuer Ventilationseinrichtungen", Das Papier 1985, Heft 10A; PA1 Lindberg, Juppi, Eskelinen, "High Speed Dryer Section Developments for Sheet Stability", 78th Annual Meeting, Technical Section CPPA, 1992. PA1 a1) a paper web which has been dewatered to a first dry solids content k.sub.1 from about 35% to about 60% by pressing is dried in a forward dryer section to a second dry solids content k.sub.2 from about 90% to about 99%; PA1 a2) the forward-drying in step a1) is carried out by applying drying energy to the paper web to be dried over the entire length of the forward dryer section asymmetrically in the z-direction from the side of a bottom face of the web; PA1 a3) the step a1) is carried out by means of a number of successive groups with single-wire draw that are open downward on support of a drying wire such that shrinkage of the web both in the machine direction and in the cross direction is reduced or at least partially prevented, which shrinkage tends to take place when the dry solids content becomes higher; PA1 b1) the paper web, which has a tendency of curling because of the asymmetric forward-drying that has taken place in steps a1)-a3), is passed to a finishing section; and PA1 b2) the paper web is finished in the finishing section by surface treatment operations, which include surface coating/sizing which are effective to moisten the web, and/or by calendaring, i.e., supercalendering and soft calendaring, which both plastically deform or "work" the web, so that the tendency of curling that arose in the web in the forward dryer section is substantially reduced. The moistening process and the plastic deformation process, if both are utilized, are typically performed one after the other in the running direction of the web.
With respect to the prior art closely related to the invention, reference is further made to the current assignee's Finnish Patent No. 91,900 (corresponding to U.S. patent application Ser. No. 07/808,161, now U.S. Pat. No. 5,416,980, the specification of which is hereby incorporated by reference herein), in which a method is described for drying a web in the dryer section of a paper machine, in particular for reducing the tendency of curling of the paper web. In the method described in FI '900, the paper web is dried on drying cylinders, against whose heated faces the paper web is pressed by means of a drying wire. In the dryer section, groups of drying cylinders are used, in which twin-wire draw and/or single-wire draw is/are applied. In this method, among other things, it has been considered inventive that in the dryer section, hot water vapor is fed substantially onto the entire width of the paper web. By means of this vapor, tensions that have been formed or that tend to be formed in the fiber mesh in the paper web are relaxed by means of heat and moisture in the area of their formation or substantially immediately thereafter.
In the prior art, a dryer section is known which is composed exclusively of the above drying groups with a single-wire draw. In these groups, between the contact-drying cylinders placed in the upper rows in the groups, normal small diameter suction rolls that are provided with inside (internal) suction boxes have been used. One particular prior art dryer section of interest is a dryer section supplied by, e.g., J. M. Voith GmbH, and situated at PM 1, Stora Feldmuehle, Reisholz, Duesseldorf, Germany which initially contained only single-tiered groups in the predryer section but later was modified to provide a double felted group as the last group in the predryer section.
A drawback of these small diameter suction rolls is the high requirement of negative pressure and suction energy because, owing to the small diameter of these rolls, high centrifugal forces arise on these rolls which tend to separate the web from the drying wire. By means of the curve sectors of small radius, the suction rolls also produce a rather large relative difference in speed between the drying wire and the web, which is in many respects unfavorable. Further drawbacks include the wear of the seals at the suction box inside the suction rolls and the repeated requirement of servicing of these seals as well as the high noise level. In this prior art paper machine, the overall concept in accordance with the present invention has not been taken into use, which concept also includes the paper finishing stages, such as surface-sizing, coating and/or calendaring.
With regard to additional prior art related to the present invention, reference is made to the current assignee's Finnish Patent No. 83,441, Finnish Patent No. 91,899 (corresponding to the current assignee's U.S. patent application Ser. No. 08/230,059, the specification of which is hereby incorporated by reference herein), to Finnish Patent Application Nos. 934367 and 935340 (corresponding to the current assignee's U.S. patent application Ser. Nos. 08/213,148 and 08/229,471, respectively, the specifications of which are hereby incorporated by reference herein), to EP Patent No. 0 427 887 and U.S. Pat. No. 5,269,074 assigned to Beloit Corporation. In the '074 patent, a dryer section is described whose initial part consists of a number of successive normal groups with single-wire draw and in whose final end there is one group with twin-wire draw in which the web has open draws between the rows of cylinders placed one above the other.
In the following, a condensed discussion will be made of the problems and requirements of further development that have been noticed in the prior art dryer sections, which have become known, e.g., from the above-mentioned patents and journal articles. As background, it should be ascertained that the highest web speeds in paper machines are, at present, already of an order of about 25 meters per second, but before long even the speed range of 25-40 m/s will be applied. Then, the bottle-neck of the runnability of a paper machine will increasingly consist of the dryer section, thus provoking an intensive effort to develop dryer sections capable of running at ever more increasing web running speeds.
In the inverted drying groups mentioned above, one particular problem is the removal of broke in the event of web breaks, for inverted groups are not self-cleaning by the effect of gravity. Thus, it is an object of the present invention to provide a paper machine, in particular for the manufacture of fine paper, in which inverted groups are not needed at all but which, nevertheless, meets the other requirements that are imposed.
The above problems and some other problems are emphasized further if, in single-wire groups, the prior art small-diameter suction rolls proper are used which are provided with inside suction boxes. In order to eliminate this problem, in some machines, it has been even necessary to open some group gaps and to lower the vacuum level in the suction rolls.
From operational experience, it is known that, if the paper is dried one-sidedly, the consequence is a tendency of curling of the sheet which is not a desired trait.
When paper is dried by means of normal groups with single-wire draw from the side of its bottom face and if such asymmetric drying is extended over the entire length of the forward dryer section, the drying takes place so that first the side of the bottom face of the paper web is dried, and when the drying makes progress, the drying effect is also spread to the side of the top face of the paper web. Thus, the dried paper is usually curled so that it becomes concave when seen from above.
As known in the prior art, the tendency of curling of paper is already affected in connection with the web formation, in particular in the stage of sheet formation (see for example, the current assignee's Sym-Former.TM. concept) by means of the discharge jet and by selection of the difference in speed of the wire as well as by means of other running parameters. As known from the prior art, for example in the case of copy paper, by means of unequalsidedness of drying in the finishing-dryer section, a suitable initial curling is regulated for the sheet in order that the curling of the paper after one-sided and two-sided copying could be optimized. In the case of copying paper, the reactivity of curling, i.e., the extent of curling produced per unit of change in moisture content, is affected to a greater extent by means of the z-directional structure, i.e., the structure in the direction of the thickness of the paper, which is produced in connection with the web formation in the wet end.
The most recent prior art related to the present invention in high-speed paper machines, in particular in fine-paper machines, has been based on dryer sections in which there is single-wire draw over the major part of the length of the dryer section and, with a view toward controlling the tendency of curling of paper, in practice, also an inverted group has always been employed in order that the drying could be made sufficiently symmetric in the z-direction. As noted above, an inverted group results in evident drawbacks in respect of the overall efficiency of the machine, as well as the runnability of the machine, and in respect of the profitability of the paper machine investment. Thus, in view of the optimization of the efficiency of a paper machine, a fully supported dryer section based on normal groups with single-wire draw, without inverted groups or groups with twin-wire draw, would be a particularly justified and advantageous embodiment. The professionals in this field have, however, not been courageous enough to develop and introduce such an embodiment because it has been considered that it would be uncontrollable and unfavorable in view of the tendency of curling of the paper.
Reference is also made to Finnish Patent Application No. 940749, corresponding to U.S. patent application Ser. No. 08/389,952, the specification of which is hereby incorporated by reference herein. This application describes a dryer section which does not include any inverted dryer groups.